Device for tuning wavelength response of an optical fiber grating

Abstract

A tunable fiber grating comprises a temperature-sensitive body secured to a fiber having a fiber grating region for transmitting thermally-induced strain to the grating. The amount of strain and hence the degree of wavelength tuning are controlled by adjusting the temperature of the temperature-sensitive body, wherein the extent of adjustment is preferably pre-determined according to feedback from a wavelength detector. Large thermal strains obtainable with the present invention allow a wide range of wavelength tuning with a relatively small and convenient temperature change near ambient temperature. In a preferred embodiment, the temperature-sensitive body is cylindrical and comprised of a nickel-titanium alloy bonded to the grating. In alternative arrangements, the thermal strain effect can be amplified. An add/drop multiplexer employing the tunable gratings is also described.

What is claimed is: What is claimed is: 
     
       1. A device for tuning the wavelength response of an optical fiber having at least one Bragg grating region with spaced-apart perturbations, the device comprising: a temperature-sensitive body having a coefficient of thermal expansion attached to the optical fiber adjacent the Bragg grating region for transmitting strain to the fiber wherein the coefficient of thermal expansion is sufficiently large to allow for tuning the wavelength response to a plurality of wavelengths with a temperature change of less than 30° C.; and   a heating element, wherein the heating element adjusts the temperature of the temperature-sensitive body to cause the body to expand or contract so that strain is induced on the fiber, thereby changing the spacing between the perturbations of the grating region.   
     
     
       2. The device according to claim 1, in which the heating element comprises a power source coupled to the temperature-sensitive body for sending electrical current to the temperature-sensitive body so that its temperature is adjusted through resistance. 
     
     
       3. The device according to claim 1, in which the temperature-sensitive body transmits a compressive strain to the fiber. 
     
     
       4. The device according to claim 1, in which the temperature-sensitive body transmits a tensile strain to the fiber. 
     
     
       5. The device according to claim 1, in which the temperature-sensitive body is cylindrical and secured to the fiber outside the Bragg grating region. 
     
     
       6. The device according to claim 1, in which the temperature-sensitive body is fabricated with an alloy selected from the group consisting of Ni--Ti, Cu--Zn--Si, Cu--Al--Ni, and Cu--Sn. 
     
     
       7. The device according to claim 6, in which the temperature-sensitive body is comprised of about forty-eight to sixty-four weight percent nickel and about thirty-six to fifty-two percent titanium. 
     
     
       8. The device according to claim 1, in which the fiber is pre-strained and tuning is achieved by controlled heating or cooling. 
     
     
       9. The device according to claim 1, in which the temperature-sensitive body is fabricated with a material having a phase transformation range of plus or minus 100 degrees Centigrade, wherein the phase transformation range overlaps with ambient temperature. 
     
     
       10. The device according to claim 1, in which the length of the temperature-sensitive body is greater than the length of the grating, region for amplifying the strain transmitted to the fiber. 
     
     
       11. A device for tuning the wavelength response of an optical fiber having at least one Bragg grating region with spaced-apart perturbations, the device comprising: a temperature-sensitive body having a coefficient of thermal expansion attached to the optical fiber adjacent the Bragg grating region for transmitting strain to the fiber;   a heating element, wherein the heating element adjusts the temperature of the temperature-sensitive body to cause the body to expand or contract so that strain is induced on the fiber, thereby changing the spacing between the perturbations of the grating region; and   a substrate having a coefficient of thermal expansion different from that of the temperature-sensitive body, wherein the length of the substrate is at least as long as the combined lengths of the grating region and the temperature-sensitive body.   
     
     
       12. The device according to claim 1, in which the temperature-sensitive body comprises a stack of a plurality of temperature-sensitive and temperature-insensitive layers, wherein the temperature-sensitive layers have a first coefficient of thermal expansion and the temperature-insensitive layers have a second coefficient of thermal expansion, and wherein the temperature-sensitive and temperature-insensitive layers are arranged so that they alternate and are interconnected. 
     
     
       13. The device according to claim 12, in which the second coefficient of thermal expansion is either zero or an opposite sign as compared with the first coefficient of thermal expansion. 
     
     
       14. The device according to claim 1, further comprising a feedback system for detecting the wavelength response of the grating and automatically adjusting the temperature of the temperature-sensitive body. 
     
     
       15. The device according to claim 1, further comprising a plurality of optical fibers with gratings secured to the temperature-sensitive body. 
     
     
       16. An improved optical multiplexer-demultiplexer device of the type having at least one pair of optical circulators and at least one optical fiber having at least one Bragg grating region with spaced-apart perturbations interconnected between the at least one pair of circulators, the improvement comprising a device for tuning the wavelength response of the optical fiber, the device comprising: a temperature-sensitive body having a coefficient of thermal expansion attached to the optical fiber adjacent the Bragg grating region for transmitting strain to the fiber; and   a heating element, wherein the heating element adjusts the temperature of the temperature-sensitive body to cause the body to expand or contract so that strain is induced on the fiber, thereby changing the spacing between the perturbations of the grating region.   
     
     
       17. An improved device according to claim 16, wherein the multiplexer-demultiplexer is an N-channel optical ADD/DROP multiplexer-demultiplexer device comprising a plurality of pairs of optical circulators and a plurality of optical fiber gratings, wherein at least one optical fiber grating is disposed between each pair of optical circulators and a device according to claim 1 is disposed adjacent each optical fiber grating. 
     
     
       18. A tunable optical fiber Bragg grating device, comprising: a length of optical fiber having a Bragg grating along a portion of its length;   a temperature-sensitive body having a coefficient of thermal expansion, the body being disposed alongside the length of optical fiber so that the body will impose a strain upon the fiber of greater than 0.05 percent for a temperature change of about ten degrees Centigrade;   a heating element for controllably adjusting the temperature of the temperature-sensitive body to transmit strain to the fiber and change the wavelength response of the Bragg grating; and,   a feedback system for detecting the wavelength response of the grating and automatically adjusting the temperature of the temperature-sensitive body.   
     
     
       19. The tunable optical fiber Bragg grating device of claim 18, wherein the temperature-sensitive body is fabricated with a material having a coefficient of thermal expansion with a magnitude of approximately 200×10 -6  /° C. 
     
     
       20. The tunable grating device of claim 18 wherein the temperature-sensitive body is comprised of alternating and interconnected layers of materials for imposing the strain of greater than 0.05 percent for a temperature change of about ten degrees Centigrade. 
     
     
       21. A device for tuning the wavelength response of an optical fiber having at least one Bragg grating region with spaced-apart perturbations, the device comprising: a temperature-sensitive body having a coefficient of thermal expansion attached to the optical fiber adjacent the Bragg grating region for transmitting strain to the fiber, wherein the temperature-sensitive body is fabricated with a material having a coefficient of thermal expansion with a magnitude of at least 30×10 -6  /° C.; and   a heating element, wherein the heating element adjusts the temperature of the temperature-sensitive body to cause the body to expand or contract so that strain is induced on the fiber, thereby changing the spacing between the perturbations of the grating region.